Host apparatus to independently schedule maintenance operations for respective virtual block devices in the flash memory dependent on information received from a memory controller

ABSTRACT

Hierarchical address virtualization within a memory controller and configurable block device allocation are disclosed. Respective virtual block devices (VBDs) are defined in flash memory managed by a common memory controller, with data access managed using address virtualization techniques. The common memory controller then tracks the need for maintenance operations independently for each VBD. Information may be received from the common memory controller regarding the need for maintenance operations in respective virtual block devices (VBDs), and commands are then selectively issued to the common memory controller in a manner so as to independently schedule these operations for the respective VBDs; performance of maintenance operations by the memory controller in a first VBD is unconstrained by performance characteristics associated with a second VBD.

PRIORITY/INCORPORATION BY REFERENCE

This application is a continuation of U.S. Utility patent applicationSer. No. 14/880,529, filed on Oct. 12, 2015 on behalf of first-namedinventor Robert Lercari for “Expositive Flash Memory Control.” U.S.Utility patent application Ser. No. 14/880,529 in turn claims thebenefit of: U.S. Provisional Patent Application No. 62/199,969, filed onJul. 31, 2015 on behalf of first-named inventor Robert Lercari for“Expositive Flash Memory Control;” U.S. Provisional Patent ApplicationNo. 62/194,172, filed on Jul. 17, 2015 on behalf of first-named inventorRobert Lercari for “Techniques for Memory Controller Configuration;” andU.S. Provisional Patent Application No. 62/063,357, filed on Oct. 13,2014 on behalf of first-named inventor Robert Lercari for “Techniquesfor Memory Controller Configuration.” U.S. Utility patent applicationSer. No. 14/880,529 is also a continuation in-part of U.S. Utilitypatent application Ser. No. 14/848,273, filed on Sep. 8, 2015 on behalfof first-named inventor Andrey V. Kuzmin for “Techniques for DataMigration Based On Per-Data Metrics and Memory Degradation,” which inturn claims the benefit of U.S. Provisional Patent Application No.62/048,162, filed on Sep. 9, 2014 on behalf of first-named inventorAndrey V. Kuzmin for “Techniques for Data Migration Based On Per-DataMetrics and Memory Degradation.” The foregoing patent applications arehereby incorporated by reference, as are U.S. Patent Publication2014/0215129, for “Cooperative Flash Memory Control,” and U.S. Utilitypatent application Ser. No. 14/047,193, filed on Oct. 7, 2013 on behalfof first-named inventor Andrey V. Kuzmin for “Multi-Array OperationSupport And Related Devices, Systems And Software” (issued on Jan. 25,2016 as U.S. Pat. No. 9,229,854).

TECHNICAL FIELD

The disclosure herein relates to non-volatile data storage and retrievalwithin semiconductor memory.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements.

FIG. 1 illustrates an embodiment of a multi-modal flash memory deviceand its interconnection to a host system.

FIG. 2 illustrates an exemplary application and configuration of a flashdevice having a pseudo-expositive memory controller within a hostsystem.

FIG. 3 illustrates an exemplary flash memory device in which discreteblock devices may be configured and allocated as described in FIG. 2.

FIG. 4 illustrates an exemplary block device allocation andconfiguration within the flash device of FIG. 3 effected using the blockdevice allocator described in reference to FIG. 2.

FIG. 5 illustrates a host perspective of the exemplary block deviceallocations and configurations presented in FIG. 4.

FIG. 6 illustrates an exemplary pair of block device configurations.

FIG. 7 illustrates exemplary generation of a physical block address inresponse to an incoming LBA.

FIG. 8 illustrates a conceptual implementation of an address generationmodule.

FIG. 9 illustrates exemplary virtualization of erase units within afour-die block device.

FIG. 10 demonstrates an exemplary sequence of operations coordinatedbetween a pseudo-expositive flash memory controller and a host fileserver.

FIG. 11 illustrates forward (and reverse) compatibility betweensuccessive flash generations enabled by a pseudo-expositive flasharchitecture provided by the teachings herein.

FIG. 12 illustrates detail regarding pseudo-physical geometry exportoptions within a flash device having multi-plane flash dies.

FIG. 13 illustrates exemplary configurations of a memory die (“Die 0”)in various embodiments.

FIG. 14 illustrates a memory controller integrated circuit (IC).

The subject matter defined by the enumerated claims may be betterunderstood by referring to the following detailed description, whichshould be read in conjunction with the accompanying drawings. Thisdescription of one or more particular embodiments, set out below toenable one to build and use various implementations of the technologyset forth by the claims, is not intended to limit the enumerated claims,but to exemplify their application to certain methods and devices. Thedescription set out below exemplifies addressing schemes and supporting,methods, devices, structures and systems. Addressing schemes and methodscan be practiced in one embodiment by a host, in another embodiment by amemory controller (e.g., within a single drive or across multipledrives), in another embodiment by a flash memory device (e.g., die orintegrated circuit) and in yet another embodiment by a host or memorycontroller cooperating with one or more other circuits. This disclosurealso provides improved designs for a memory controller, host, memorydevices, a memory system, a subsystem (such as a drive, e.g., a solidstate drive or “SSD”), and associated circuitry, firmware and addressingmethodology. The disclosed designs can also be implemented in softwareor instructions for fabricating an integrated circuit (e.g., a circuitdesign file or field programmable gate array or “FPGA” configuration).While the specific examples are presented, particularly in the contextof flash memory, the principles described herein may also be applied toother methods, devices and systems as well.

DETAILED DESCRIPTION

A memory controller that subdivides an incoming memory address intomultiple discrete address fields corresponding to respectivehierarchical groups of structural elements within a target nonvolatilesemiconductor memory system and in which at least one of the discreteaddress fields constitutes a virtual address for the correspondingphysical element within the structural hierarchy, is disclosed invarious embodiments. Through this hierarchical subdivision, the virtualaddress portion of the incoming memory address is ensured to resolve toan element within the physical bounds of a larger(hierarchically-superior) structure, but may be freely mapped to any ofthe constituent physical elements of that larger structure. Accordingly,a host requestor may issue logical memory addresses with address fieldspurposefully specified to direct read, write and maintenance operationsto physically distinct structures within the memory system in a mannerthat limits performance-degrading conflicts while the memory controllerremains free, by virtue of one or more virtualized address fields withinthe incoming logical addresses, to virtualize localized groups ofphysical structures and thus mask defective structural elements and swapoperational structural elements into and out of service, for example, asthey wear or otherwise require maintenance.

In other embodiments presented herein, the net storage volume of anonvolatile semiconductor memory system is subdivided into discreteperformance-isolated storage regions based on specified systemrequirements and underlying memory system geometry and performancecharacteristics, with each such storage region being mapped by anindependent linear range of logical addresses. Accordingly, eachperformance-isolated storage region may be presented to one or more hostaccess requestors as an independent block device (i.e., mass storageunit having a continuously mapped logical address space) so that thenonvolatile memory system may be perceived by that host as beingconstituted by multiple discrete block devices, each having its ownperformance characteristics and address space. Moreover, the mapping ofthe logical address space within a given block device, referred toherein as “address space layout,” may vary from one block device toanother (e.g., sequential addresses within logical address ranges ofrespective block devices may be distributed within the structuralhierarchy of the memory system in different order) to yield configurableand varied block device characteristics in terms of endurance and I/Obandwidth. Further, multiple different address space layouts may beapplied within different “subspaces” of a given block device (i.e.,discrete portions of the block device's address range) with, forexample, addresses in one subspace being sequentially applied tostructural elements at different hierarchical levels of the memorysystem in a different order than in another subspace. Also, in a numberof embodiments, system requirements specified (e.g., by a user/systemdesigner) in terms of block device capacity and performance metricsincluding, without limitation, read and write bandwidth requirements andminimum data transfer size required by the block device, areautomatically translated into corresponding configuration and allocationof structural elements as necessary to meet the high-level requirements,with such configuration and allocation optionally being programmeddirectly into the nonvolatile memory subsystem and/or correspondingblock device definition reported to a host access requestor. By thisapproach, a system designer may configure and allocate block devicesaccording to performance requirements of the application at hand withouthaving to resort to the complex and error-prone task of allocating andconfiguring numerous physical resources within the nonvolatile memorysystem individually. Moreover, in a number of embodiments, high-levelperformance requirements specified to configure and allocate blockdevices within a given memory subsystem may be used to enableforward-compatible allocation and configuration of like-performanceblock devices within next-generation memory subsystems and thus enableseamless memory subsystem replacement (with or without data migration)and/or supplementation with next-generation technologies. These andother embodiments, features and benefits are described in greater detailbelow in reference to exemplary drawing figures.

The nonvolatile memory subsystem in which a number of embodiments aredetailed herein is presented as a flash memory device forming in wholeor part a solid state disk (SSD) drive; the flash memory device can behierarchically arranged in multiple wired signaling channels eachcoupled to multiple flash memory dies, with each die including numerousindividually erasable storage units (“erase units” or erase blocks orflash blocks) distributed in one or more access planes, and with eacherase unit including numerous pages constituted by a predeterminednumber of single-bit or multi-bit nonvolatile storage cells (i.e.,channels, dies, erase units and pages constitute, for example andwithout limitation, respective hierarchical physical elements within theflash memory device). For example, in one embodiment, a memorycontroller within the flash memory system (e.g., within the drive orSSD) subdivides each incoming “logical block address” (LBA) intorespective channel, die, erase unit and page address fields, any or allof which may be virtual addresses, and resolves a commanded read orwrite access to a specific channel indicated by the channel addressfield, a specific die indicated by the die address field, a specificerase unit indicated by the erase unit field (including possibleresolution to two or more erase units in the case of multi-plane commandsequences) and a specific page indicated by the page address field(including possible resolution to two or more pages in the case of amulti-page operation).

Numerous specific details relating to flash memory device technologyincluding, for example and without limitation, erase granularity limitedto whole erase units (a fundamental characteristic of flash memory),usage-induced storage cell wear (i.e., as program and erase cyclesinvolve destructive charge-carrier passage through isolating oxidelayers) and finite storage cell retention times (largely due to leakagefrom floating storage cells which increases as feature sizes shrink insuccessive device generations) and disparate data write (or “program”)timing and read timing constraints, bear on implementation and/oroperational details of a number of embodiments presented below. In allcases, such details may change or be omitted when not required by theunderlying memory technology. Thus, while the various embodimentspresented herein are described in the context of flash memory devices,the structures and techniques disclosed are not so limited and may beapplied with respect to any underlying memory technology whereverbeneficial including both nonvolatile and volatile memory technologies.

FIG. 1 illustrates an embodiment of a multi-modal flash memory device(“flash device”) and its interconnection to a host system or device,referred to herein as the “host.” As shown, the flash device includesmultiple flash memory dies coupled to a multi-modal flash memorycontroller (MC). It should be understood that the memory controller isdepicted in multiple locations within the figure, to illustratedifferent modes of operation; the memory controller itself includes amode register or other configuration circuit which, when programmed bythe host or pre-programmed during or after flash device production,enables the flash device to be selectively operated in any one of atleast four different memory control modes including a physical accessmode 112, a linearly virtualized mode 114, a cooperative management mode116, and a hierarchically virtualized mode 118.

In physical access mode, shown generally at 112, the flash device takeslittle or no autonomous action and instead merely executes read, writeand erase requests at the direction of the host. Thus, the host is fullyaware of the underlying flash device geometry (shown, in this conceptualexample, as three flash memory dies each having five erase units) andissues a physical block address (PBA) with each read/write request—thatis a memory address having a fixed, one-for-one correspondence to alogical storage block (e.g., smallest addressable unit of data storage)within the collective storage space formed by the constituent flash diesof the flash device. An address received by the memory controller fromthe host is substantially identical to the address transmitted from thememory controller to memory; that is, the physical access mode enablesexecutions of flash memory input/output operations (IOPs) withtheoretically minimal latency (i.e., no address translation is neededand the host can schedule IOPs in a manner that avoids resourceconflicts due to full awareness of the underlying physical hardware andlimitations thereof). In this mode, however, the host is burdened withnumerous complex and hardware-specific media management tasks, includingdiscovery and avoidance of failed structural elements (especially eraseunits and dies), leveling otherwise disparate wear between differenterase units (“wear leveling”), reducing storage fragmentation (“garbagecollection”) as the ratio of partially filled erase units to availablecontinuous storage space rises, and refreshing (i.e., re-writing in anew location) data nearing its retention time limit (“scrubbing” ageddata). Thus design and implementation of a host system needed tointeract with and manage the flash memory in physical access mode canbecome tremendously complex and, making matters worse, may requiresubstantial and expensive re-design as new generations of flash memorydevices become available.

Still referring to FIG. 1, if physical access mode represents an extremeburdening of the host in return for potentially minimized IOP latency,the linearly virtualized mode shown at 114 represents the oppositeextreme. In linearly virtualized mode, a flash translation layer (FTL)and media management engine are enabled within the memory controller topresent the aggregate storage of the flash device (i.e., collectivestorage area within the three flash memory dies) to the host as a simplelinear address range in which all media management operations areexecuted transparently (i.e., without host visibility orresponsibility). While the FTL and under-hood media managementtremendously simplifies host design and implementation, the cost of thatsimplicity is a substantial degradation of flash device performance andcapacity. For example, because every logical block address issued by thehost (e.g., LBA i−1, LBA i, LBA i+1, LBA i+2 as shown in FIG. 1) may mapto any storage location within the flash device (e.g., any page withinany erase unit within any die on any signaling channel within the flashdevice), translation time tends to be substantial and increasesexponentially as flash capacity grows (i.e., in new generations of flashdevices). Worse, because the mapping of logical block addresses tounderlying storage locations is invisible to the host, the host isunable to avoid sequential issuance of logical block addresses thatresolve to the same flash die, meaning that resource conflicts may ariseat random. From the host perspective, the I/O latency (i.e., timerequired to complete an I/O) is simply unpredictable and may, at anyinstant, grow many times beyond the native operational time imposed bythe underlying flash die hardware. This unpredictable latency is madeeven worse by the memory controller's autonomous scheduling of flashmaintenance operations (i.e., garbage collection, wear-leveling, datascrubbing) each of which tends to involve numerousmemory-controller-initiated write operations followed by one or morerelatively slow erase operations and thus may dramatically increase theI/O latency perceived by the host.

Continuing with the linearly virtualized controller mode 114, the memorycontroller's responsibility for flash maintenance requires that it keepsubstantial storage regions in reserve. Even more space is typicallyreserved to reduce the probability of worst-case resource conflictscenarios (i.e., limit occurrence of system-buckling long-latency I/Oevents). As a result, the overall flash memory capacity made availableto the host in linearly-virtualized mode (i.e., the space encompassed bythe linear LBA range) is generally substantially less than the physicalcapacity of the device. This “overprovisioning” of the physical storagespace to meet a host-desired capacity and performance is exacerbated bythe need for nonvolatile storage of the ever-growing FTL translationtable (i.e., growing as the flash device capacity is consumed, andgrowing with new generations of more capacious flash devices) within thenonvolatile flash storage itself.

Still referring to FIG. 1, the cooperative management mode shown at 116achieves the performance advantages of physical access mode by exposingthe underlying flash geometry to the host, but limits the host's flashmanagement burden by gathering statistics needed to manage flashmaintenance operations and by notifying the host when maintenance isrequired. The general approach is enumerated in the detail view ofcooperative management mode and in which the host (1) requests andreceives a physical geometry description from the memory controller(referred to herein as a “geometry export”) and then (2) defines anaddress assignment in which the linear block address range of the hostfile system is mapped directly onto the physical block address withinthe underlying flash storage. In the particular example shown, adefective and unavailable erase unit marked by ‘B’ is reported to thehost by the memory controller and noted within the LBA-to-PBA addressmap.

Hierarchically virtualized mode, shown for example at 118, takescooperative management mode a significant step forward by presenting tothe host an idealized view of underlying physical structures within theflash memory device. That is, as in cooperative management mode, thehost requests a physical geometry description from the memorycontroller, but the memory controller returns, instead of a truephysical description with all attendant details regarding defectivestorage and other realities, an idealized or pseudo-physical descriptionof the underlying geometry that enables abstraction of the underlyingflash memory structures without loss of coherence with respect toboundaries between hierarchical structures. Thus, in thehierarchically-virtualized example shown, the memory controller informsthe host that the aggregate flash storage is subdivided among threeflash dies and that four erase units are available within each flashdie, holding in reserve some number of erase units (or dies or any otherresource likely to fail over time) as necessary to maintain a staticidealized perspective of the underlying flash geometry for the host.Accordingly, despite the defective erase unit (‘B’) within the centerflash die or even run-time detected failure of erase units within theother two flash dies, the host perceives a defect free set of physicallyextant dies and erase units therein. This pseudo-physical hostperspective may be appreciated by comparing the host-perceived flashdevice architecture (three defect-free dies, each having four eraseunits) shown by the LBA to pseudo-physical block address (PPBA) mappingat 121 with the memory controller mapping of the LBAs to underlyingphysical block addresses (PBAs) as shown at 123. Whereas the hostperceives a linear address mapping to the idealized physical storage(i.e., pseudo-physical storage), the memory controller maps the LBAsdiscontiguously, skipping over reserved and defective erase units, andthus virtualizing the pool of erase units within each individual flashdie while maintaining the physical boundary between dies as reported tothe host. Contrasting the cooperative management and hierarchicallyvirtualized operating modes shown in FIG. 1, the controller fullyexposes the physical flash hardware in the cooperative management modeand exposes a pseudo-physical hardware set in the hierarchicallyvirtualized mode and is thus referred to herein alternately as anexpositive or pseudo-expositive memory controller when operating inthose respective modes, with the pseudo-expositive memory controllerbeing a specialized instance of the more general expositive memorycontroller.

Note that in the cooperative management mode 116 and the hierarchicallyvirtualized mode 118, some limited amount of address translation can beperformed at the memory controller, e.g., by translating the address ofone block in the hierarchy (e.g., erase unit) while preserving logicallocation level at other address levels (e.g., preserving page orderingwithin a remapped erase unit); in the cooperative management mode 116,such remapping can be temporary (e.g., with the host ultimately beinginformed for example of bad block remappings, for example), and in thehierarchically virtualized mode 118, such remapping can be transparent,with a memory controller deriving for example, any of erase unit, page,die, plane, device, channel or other hierarchical address distinctionswhile preserving address-space division (e.g., logical order) at otherlevels. Among other advantages, this architecture provides for greatlysimplified address translation (e.g., which can optionally beimplemented entirely in hardware), and facilitates configurable andpredictable I/O latency, and greatly shortens address translation timeand associated complexity.

FIG. 2 illustrates an exemplary application and configuration of a flashdevice having a pseudo-expositive memory controller (i.e., multi-modememory controller operating in a pseudo-expositive mode, or memorycontroller having only a pseudo-expositive operating mode) within a hostsystem. In the particular embodiment shown, an interactive block-devicedesigner/allocator is executed within either the flash device, hostsystem and/or separate computing device to obtain flash device geometryand performance characteristics and interact with a system designer(i.e., user, system architect or other human operator or programmedintelligence) to define and allocate one or more discreteperformance-isolated storage regions within the flash device. The blockdevice designer/allocator (“block device allocator” for short) suppliesparameterized descriptions of each allocated flash storage region to ahost file system (i.e., one or more threads or processes executed on ahost computing system to carry out file storage, retrieval andmaintenance operations with respect to files and other data objects)which, accordingly perceives each such flash storage region as a linearrange of logical block addresses and thus as an independent blockdevice.

Still referring to FIG. 2, and more specifically to the exemplaryoperational flow at 151, the block device allocator initially requestsand receives geometry and performance data from the flash device and/ora device library at 155. In the embodiment shown and a number ofembodiments described below, the geometry information includeshierarchical topology information including, for example and withoutlimitation, (1) the number of independent flash-die access channels(also referred to herein as “signaling channels” or simply “channels”)coupled to or otherwise accessible by the memory controller, (2) thenumber of discrete flash dies coupled to each access channel, (3) thenumber of erase-unit planes (or “planes” for short) within each flashdie, (4) the number of erase units within each plane, (5) the number ofpages per erase unit and (6) the storage capacity of each page. Ingeneral, each of the device geometry parameters may be a physical (i.e.,actual) or pseudo-physical representation of the correspondingunderlying hierarchical structure (e.g., channels, dies, planes, eraseunits or pages). Performance data, which may be viewed as part of thegeometry export, may include for example and without limitation, (a) thenominal programming time of a physical page within the erase unit of aflash die (i.e., “t_(PROG) ^(”) also referred to herein as the pagewrite time), (b) the nominal read time of the physical page (t_(READ)),(c) the nominal time required to erase contents within an erase unit(t_(ERASE)), (d) the data transfer bandwidth over an individual accesschannel (Chan BW), (e) the data transfer bandwidth of the host interface(H-Intf BW), (f) various performance enhancement/optimization functionsthat may be available (e.g., multi-page functionality, read caching,write caching, etc.), and so forth.

At 157, the block device allocator determines the available block deviceprofiles (i.e., physical and performance characteristics of differentconfigurations of physical resources within the flash device) accountingfor any pre-existing allocations. Assuming that no block devices havebeen allocated at this point and thus that resources sufficient forfurther block device definition remain within the flash device (i.e.,negative determination at decision 159), then the block device allocatordisplays (e.g., in a visible display or other user interface of thecomputing device in which the block device allocator is instantiated)available block device profiles and the allocable quantities of each asshown at 161, prompting the user/designer to select one or more blockdevice profiles and their desired allocation quantities. Upon receivinguser input specifying a quantity of block devices having a given profile(and possibly multiple different profiles and respective quantities) at163, the block device allocator prompts the user/designer to specify,for each block device to be allocated, whether the LBA range of theblock device is to be uniformly sequenced among the hierarchicalstructures of the flash storage region to be allocated (e.g., channels,dies, erase units, pages) thus establishing a uniform address spacelayout (ASL) for the block device, or whether the address space layoutis to be varied within one or more “subspaces” of the block device(i.e., “sub-ranges” of the overall LBA range for the block device). Ifsubspace ASL is desired (affirmative determination at 165), the user isprompted to specify one or more specialized LBA ranges (i.e., LBA rangeshaving ASLs different from the otherwise uniform ASL corresponding tothe block device profile). After receiving subspace ASL specifications,if any, for each user-requested block device, the block device allocatorprograms block device configuration registers 169 within the flashdevice (thereby allocating individually configured block devices withinthe flash device) and notifies the host file system of the block deviceallocation, supplying, for example a parameterized description of eachblock device that informs the host file system of the block devicecapacity (thus establishing the LBA range of the block device for agiven LBA granularity), write bandwidth, read bandwidth and minimum datatransfer size.

Note that the LBA granularity (i.e., size of a logical block of data—notto be confused with a block device which will typically hold millions oflogical blocks of data each having a respective LBA) may be programmedwithin the block device configuration registers or other configurationstorage of the flash device to enable a variable user-specified numberof LBAs to span each physical page of storage within the flash device.Similarly, as discussed in greater detail below, the size of a logicalquantum of data, referred to herein as a “host data segment” or“segment” and that constitutes a fundamental unit of storage allocationoperated upon by the host file system, may be programmed within theblock device configuration register to enable the memory controller toassociate discrete sets of physical storage structures (e.g., an integernumber of erase units within a given block device) with respectivesegments and thereby facilitate coordinated file-system and flash devicemanagement operations.

Continuing with the block device allocator flow, after programming theblock device configuration registers and exporting corresponding blockdevice parameters to the host file system at 169, the block deviceallocator returns to the space availability evaluation at 157, anddetermines the block device profiles and quantities thereof that remainavailable for allocation after accounting for any prior allocations at169. If all available block devices are determined to have beenallocated (i.e., all resources reported to be available by the flashmemory device and/or library description of the flash memory device havebeen allocated in a block device definition or respective block devicedefinitions), then the block device design/allocation operation isdeemed complete and the block device allocator terminates. Though notspecifically shown, the designer/user of the block device allocator mayalso terminate the block device allocation sequence without allocatingall flash resources.

Reflecting on the block device allocation sequence shown in FIG. 2, itshould be noted that numerous additional provisions and or options maybe implemented in various embodiments. For example, instead of executingthe flash memory/file system configuration operation at 169 in each loopiteration, the block device allocator may accumulate an internal set offlash device and flash memory configuration parameters in each loopiteration, prompting the user/designer to review a comprehensiveallocation plan before proceeding with flash device programming and/orfile system notification. Also, instead of issuing configurationcommands/parameters to the flash memory device and/or host file system,the block device allocator may write configuration information to one ormore configuration files (or other data objects) for later delivery toactual flash memory devices or host file systems. Further, instead ofiteratively prompting a user to select from among available block deviceprofiles (which selection constitutes an implied or explicituser/designer specification of block device characteristics, includinglogical block size, read bandwidth, write bandwidth, capacity, transfersize, endurance metric, etc.), the block device allocator may initiallyreceive desired block device performance parameters from a user/designerand thus proceed from the user's initial performance specification toresolve block device allocations. In yet other embodiments, the quantityof block devices and/or their individual characteristics may bedetermined in whole or part by user/designer specification of aggregateread or write bandwidth within the flash device as a whole (togetherwith other “overall device” requirements, such as a minimum segmentsize, logical block size and/or transfer size), with the block deviceallocator allocating and configuring, for example, a minimum or maximumnumber of block devices as necessary to meet those aggregaterequirements. Accordingly, the iterative block device allocationsequence shown in FIG. 2 should be understood as but one of manypossible block device allocation schemes which may be deployed indifferent embodiments or configurations.

FIG. 3 illustrates an exemplary flash memory device in which discreteblock devices may be configured and allocated as described in FIG. 2.For purposes of explanation and understanding only, a specific flashdevice geometry and set of performance metrics are shown in FIG. 3 andcarried forward in a number of embodiments described in reference tofollowing figures of the drawing. In all such cases, the specificgeometries (e.g., numbers of physical elements at each hierarchy withinthe flash device structure including, without limitation, accesschannels, flash dies per channel, planes per flash die, erase units perplane, pages per erase unit, page size, page read and write times(t_(READ) and t_(PROG)), channel bandwidth and host interface bandwidth,number of host interfaces, etc.) may be different from those shown inFIG. 3.

With that understanding, it can be seen that the exemplary flash devicein FIG. 3 includes a memory controller coupled to 16 access channels(ch0 . . . ch16), with each access channel coupled to eight flash dies,each flash die having two planes, and each plane containing 1124 eraseunits (2248 erase units per die), and each erase unit containing 256pages each containing a number of flash memory cells (e.g., NAND memorycells, though NOR or flash memory cell configurations may be deployed inalternative embodiments) sufficient to store 16 kilobytes (kB) of data.As an aside, references to volumes of data herein are generally intendedto mean a power of two quantity so that KB, MB, GB, TB, etc. referrespectively to 2¹⁰, 2²⁰, 2³⁰, 2⁴⁰, etc. byte of data (sometimesdesignated by KiB, MiB, GiB, TiB and so forth).

Continuing with the flash memory embodiment shown in FIG. 3, forpurposes of example, it is assumed that the geometry exported to theblock device allocator accurately reflects the underlyingstructural/physical hierarchy in all aspects but one—that the number oferase units per plane is reported pseudo-physically as 1024 (i.e., 2¹⁰and thus 2¹¹ or 2048 erase units per die) instead of 1124, meaning that100 erase units per plane and thus 200 erase units per die are held inreserve. As explained below, this erase unit reservation or “spare pool”enables the memory controller to virtualize the available erase unitswithin each die and thus mask defective erase units from host visibilityand swap erase units into and out of service as storage for respectiveLBA ranges of an allocated block device without impacting the hostperspective of the block device or the flash device as a whole. Also, itbears emphasis that each virtualized collection of erase units isconfined to the individual flash die (or even the individual planes,depending on block device configuration) containing the correspondingphysical erase units. That is, while a given LBA that resolves to aflash die may include a virtualized erase-unit address field (an exampleof a discrete address field within the LBA corresponding to a group ofphysical elements within a structural hierarchy—erase units in thiscase), the LBA is nonetheless guaranteed by design to resolve to aphysical erase unit with the LBA-specified physical die on theLBA-specified physical channel. More generally, as discussed in greaterdetail below, while any address fields (i.e., contiguous ordiscontiguous ranges of bits within an incoming LBA) corresponding to ahierarchical group of structures (e.g., channels, dies, erase units,pages) may be virtualized such that one or more of the correspondinghierarchical structures may be held in reserve, the address-spacedivision between the hierarchical structures is maintained. Thus, oncethe channel field of an LBA is resolved to a particular channel, it isassured that, despite possible virtualization of the sub-hierarchicalfields (i.e., die field, erase unit field and/or page field within theLBA), the LBA will ultimately resolve to a physical page on a physicaldie on that particular channel. This principle holds true for each levelof the structural hierarchy so that once the LBA is resolved to aparticular die, it is assured that the remaining erase unit and pagefields, even if one or both are virtualized, will resolve to a physicalpage within that die, and likewise an LBA resolved to a particular eraseunit is assured to specify a physical page within that erase unit,despite possible virtualization of the LBA's page field. Through thisarrangement, it becomes possible to virtualize physical structures ateach hierarchical level within the flash device while guaranteeingaccess to discrete subdivisions of the flash storage and therebypermitting, for example, specification of discrete subsets of dies andchannels (or even finer-grained hierarchical elements where it makessense to do so) to respective block devices specified within the blockdevice configuration storage.

FIG. 4 illustrates an exemplary (and not necessarily complete) blockdevice allocation and configuration within the flash device of FIG. 3effected, for example, using the block device allocator described inreference to FIG. 2. As shown, five block devices, BD0-BD4, areallocated, with each block device being constituted by one or more flashdies that are collectively associated with one or more memory channels(i.e., in this example, the smallest allocable block device is a singleflash device). The block device configuration storage (BD Config)includes a block device lookup table (BD LUT) that is indexed by a blockdevice identifier (BDid which may be, for example, a namespace or otherhandle or identifier) and holds, for example and without limitation, aspecification of the one or more channels spanned by the block device(i.e., “constituent channels”) and the one or more dies on theconstituent channels. For instance, block device 0 (BD0) spans channels0 and 1, and includes dies 0 and 1 on the constituent channels and thusincludes four flash dies in all. Block device 1 is similarly defined asencompassing a 2×2 set of flash dies, but spans channels 2 and 3 insteadof 0 and 1 and thus forms an entirely discrete storage area (i.e., nophysical overlap) with respect to block device 0. Block device 2 spans asingle channel (channel 0) and flash dies 2-5 on that channel, and thusalso constitutes a four-die block device, but without the channelparallelism of block devices 0 and 1 (i.e., a 1×4 block device insteadof a 2×2 block device). Block device 3 is also a four-die 1×4 blockdevice, but is constituted by allocation of dies 2-5 on channel 1instead of channel 0. Lastly, block device 4 is a 16-die block devicethat includes a single die (die 6) in all 16 channels—that is, a 16×1block device. Thus, of the overall set of five allocated block devices,four block devices (BD0-BD3) include discrete sets of four constituentdies and thus have a 32 GB pseudo-physical storage capacity (i.e., eachdie includes a pseudo physical count of 2048 erase units, eachcontaining 256 16 KB pages and thus 2¹¹*2⁸*2¹⁴ bytes or 8 GB) and thefifth block device (BD4) includes a discrete set of sixteen dies andthus a 128 GB pseudo physical storage capacity. Also, in this particularexample, the channel and die identifiers recorded within the blockdevice lookup table correspond one-for-one with underlying physicalchannels and dies within the flash device as neither of those elementsof the structural hierarchy are virtualized. In alternative embodimentsor configurations of the flash device, either or both of those elementsmay be virtualized.

Continuing with FIG. 4, each entry within the block device lookup tableadditionally includes an index to a respective set of one or moreentries within an ASL lookup table (ASL LUT), with each ASL lookup tableentry including ASL (address space layout) parameterizations and featurecontrol settings for the complete or partial LBA range of thecorresponding block device. In the particular example shown,ASL/feature-control parameter settings for three different block devicesare illustrated, including a uniform ASL/feature-controlparameterization within block device 0 (i.e., parameterization appliesuniformly to the entire LBA range of the block device—there are nosubspace definitions), another uniform ASL/feature-controlparameterization with block device 2, and another multi-subspaceASL/feature-control parameterization within block device 4, withseparate ASLs defined for each of multiple LBA ranges within the blockdevice, only two of which are shown. Entries for block devices 1 and 3are also present in the ASL lookup table, but not specifically shown inFIG. 4.

As discussed above, the ASL parameters define the manner in whichsequential LBAs are distributed within the structural hierarchy of theblock device and thus indicate the number of pages within the same eraseunit (i.e., “seqPg”) to which sequential LBAs apply before progressingto page(s) in the next erase unit, and then the number of erase units tobe sequentially accessed within a given die (“seqEU”) before progressingto the next die, and then the number of dies to be accessed on a givenchannel (“seqDie”) before progressing to the next channel. The featurecontrol parameters include, for example and without limitation, whetherread caching and write caching are to be enabled (independently settablevia the rdC and wrC fields of the ASL lookup table entry) for the blockdevice or subspace thereof, the number of pages that may besimultaneously or concurrently written to or read from within the sameerase unit (nPa), and the number of erase-unit planes to be concurrentlyaccessed in a given write or read command sequence (nPl). In general,read caching is a double-buffering construct that enables data retrievedfrom an address-selected storage page and stored within the flash die'spage register (i.e., a buffer element that temporarily holds outboundpage-read data and inbound page-write data) to be output from the flashdie concurrently with transfer of subsequently selected storage-pagedata to the page register, and write caching is a similardouble-buffering arrangement that enables concurrency during page-writeoperations. Thus, the read and write page caching features, whenenabled, reduce net latency of a sequence of read or write operations,respectively. In general, page caching scales (e.g., multiples accordingto cache depth) the effective size of the page register and thuscorrespondingly raises the minimum data transfer size imposed on thehost in a given page read or write operation. For simplicity ofunderstanding, page caching in both the read and write directions isdisabled (i.e., “off”) within the exemplary ASL lookup table entriesshown. Multi-page operation (i.e., nPA set to a value greater than one)and multi-plane operation (nPl set to a value greater than 1) likewiseraise the minimum data transfer size between the host and memorycontroller. In the specific examples shown in the ASL lookup table ofFIG. 4, multi-page operation is disabled at least within block devices0, 2 and 4, and multi-plane operation is disabled at least within blockdevices 2 and 4. By contrast, dual-plane operation (nPl=2) is specifiedfor block device 0, thus doubling the number of concurrently writtenpages (a block device characteristic referred to herein as a “stripesize” or “frontier width”) as compared to a physically-comparable blockdevice (e.g., block device 1) configured for single-plane operation.

Still referring to FIG. 4, the singular values (‘1’) recorded for theLBA-sequencing ASL parameters within the ASL lookup table entry forblock device 0 dictate that LBAs are to be distributed widely throughthe structural hierarchy of the flash device with, for example, eachpage of a sequentially addressed set of four pages (and thus asequential range of 16 LBAs given the sLB=4 KB logical block sizesetting) being read or written within a different one of the fourconstituent dies of the block device, thus effecting 4×(“multiplied-by-four”) parallelism in the page read or write operationand thus approximately a 4× bandwidth increase over a configuration thatdirects four sequential page read or write accesses to the same eraseunit within a given die. In actuality, the dual plane setting withinblock device 0 doubles the number of concurrently accessed pages withineach of the four constituent flash memory dies of block device 0, thusestablishing an 8-page stripe size and thus read and write bandwidthsthat are approximately eight times the baseline single page read andwrite bandwidths of a given flash die. By contrast, block device 1 whichotherwise constitutes the same 2×2 die-layout as block device 0, isconfigured with a deep (rather than wide) address space layout and withonly single-plane access throughout its LBA range. Thus, pages fallingwithin a sequential LBA range of block device 1 are mapped to physicalpages within an erase unit until the end of that erase unit is reached(i.e., “Pg/EU”), with the LBA range thereafter continuing through thephysical pages of a next erase unit and then the next until the finalerase unit (EU/Die) within the die has been traversed, with the LBArange thereafter continuing through the pages and erase units of thenext die on the same channel before carrying over to EUs and dies on anadjacent channel. In general because the stripe size is very narrow(i.e., a single page and thus as narrow as possible in the exemplaryflash memory die shown), only a single erase unit is impacted by eachpage write operation and thus, despite the relatively low writebandwidth (i.e., 16 KB/t_(PROG) in contrast to the 8*16 KB/t_(PROG)bandwidth achieved by the 8× wider stripe size in block device 0),write-induced wear resulting from, for example, storage of a 16 MB hostsegment, will occur in only a relatively small number of erase units anddies—four 4 MB erase units within the same die in this example—incontrast to the storage of each of the 1024 16 KB pages within arespective one of the 1024 erase units of block device 0 (i.e., each ofthe 256 erase units within each of the four dies is written to and thusworn to some degree). Conceptually, while the total page wear isequivalent in both block die configurations (the same number of pages iswritten), the wear within narrower-stripe-size (i.e., deeper, but lowerbandwidth) block device 1 is concentrated within a much smaller numberof dies (and potentially erase units as well), simplifying wear-levelingoperations and thus raising endurance of the block device as compared towider, higher-bandwidth block device 0. In general, all theseconsiderations are made available to the user/system designer inconfiguring the block devices, as the designer is able to directly, orwith the aid of the block device allocator, specify ASL andfeature-control parameters that strike a design balance betweenendurance and bandwidth requirements of a given application, potentiallyconfiguring block devices having widely varying performancecharacteristics within the same flash memory device, as illustrated byblock devices 0-4 in FIG. 4.

Concluding with FIG. 4, the ASL lookup table includes multiple entries(and thus multiple subspace definitions) for block device 4, with aninitial entry specifying a 4K LBA range (0-4095) mapped for high I/Obandwidth (i.e., 16-page stripe size as the write frontier spans allsixteen of the flash dies within the block device) and another entryspecifying a subsequent 4K LBA range mapped for endurance (single-pagestripe size). One or more other subspace definitions (not shown)complete the LBA range mapping of block device 4.

FIG. 5 illustrates a host perspective of the exemplary block deviceallocations and configurations presented in FIG. 4, showing relativeblock device capacities by the area/footprint of the block devices (aswill be the case within the physical flash storage media) andillustrating relative block device bandwidths according to block devicewidth (left-to-right across the drawing figure). Thus, block devices 0-3all have the same four-die, 32 GB capacity and thus have the samefootprint (i.e., spanning an equivalent area within the figure), whileblock device 4, encompassing four times as many flash dies as each ofthe other block devices has a correspondingly larger 128 GB capacity.Also, block devices 0, 1, 2, 3 and 4 are depicted with respective widthsaccording to their nominal maximum I/O bandwidths of 8×, 1×, 1×, 2× and16×, respectively, where each non-unity multiplier scales the baselineread and write bandwidths (i.e., 1/t_(READ) and 1/t_(PROG)). As shown indetail view 251, the signaling bandwidth of block device 4 variesaccording to subspace, having a high (16×) bandwidth in the initial LBArange, and a 1× bandwidth in the LBA range that follows (i.e., inaccordance with the respective subspace stripe sizes of 256 KB and 16KB). Note also that actual block device bandwidths may be constrained bypeak channel and/or host interface signaling rates, particularly in thecase of memory read operations. For example, because the host interfaceis shared between all block devices (at least in the singlehost-interface embodiment shown in FIGS. 3 and 4) and individual accesschannels may be shared by as many as eight block devices (or more orfewer according to the number of flash dies per channel), operationsexecuted in one block device may cut into the signaling bandwidth neededfor peak-rate operation within another block device, particularly wherenumerous high-bandwidth block devices share the same signaling accesschannel or a large number of block devices are allocated within theflash device (which may saturate the host interface during occasionalconcurrent read access within a threshold number of block devices).

FIG. 5 also presents a non-exhaustive list of flash device performancecharacteristics and features enabled by hierarchical addressvirtualization within the memory controller and configurable blockdevice allocation, including (i) a configurable and predictable I/Olatency, (ii) brief or otherwise negligible logical-to-physical (“L2P”)address translation time (or at least dramatically reduced L2P timerelative to operating modes that invoke a flash translation layer),(iii) allocation of discrete defect-free block devices in user-specifiedquantities and capacities, (iv) host-scheduled maintenance ofcontroller-identified segments (or other logical units of storage), and(v) forward compatibility/ease of migration as flash device capacitiesand feature sets expand in successive device generations. Each of thesecharacteristics/features is discussed in greater detail below.

FIG. 6 illustrates an exemplary pair of block device configurations thatdemonstrate the configurable and predictable I/O latency enabled by thepseudo-expositive flash device described above (i.e., flash devicehaving a pseudo-expositive memory controller) and the user-configurableblock device allocation therein. These configurations will berespectively referred to as “configuration A” and “configuration B,”with it being understood that a device may be reconfigured dynamicallyor otherwise as desired, as indicated by the presence of a migrationarrow separating these two configurations. As shown, each of the blockconfigurations features a 2×2 disposition of four dies (i.e., two diescoupled to channel 0 and two dies coupled to channel 1) and two planesof erase units within each die. That is, the block device configurationsrepresent physically identical geometry and capability. However, byprogramming a wide and shallow ASL (i.e., ASL parameters set todistribute sequential LBAs through a minimum number of physicalstructures at each level of the structural hierarchy) and enablingdual-plane operation, an 8-page I/O stripe width is implemented withblock configuration A and thus an 8 page/t_(PROG) write bandwidth and 8page/t_(READ) read bandwidth, assuming sufficient overhead within thehost-interface and flash-die access channels (as indicated by the 8 EU'sin bold in the left-most grouping of 4 dies in FIG. 6). By contrast,block device configuration B features a narrow and deep ASL (i.e., ASLparameters set to distribute sequential LBAs through a maximum number ofphysical (or pseudo-physical) structures at each level of the structuralhierarchy) and for single-plane operation, so that a single-page stripesize is established and thus write and read bandwidths of only 1page/t_(PROG) and 1 page/t_(READ), respectively. As explained above, thesequencing of LBAs through all the physical (or pseudo-physical)structures of a given die before proceeding to the next die increasesendurance of the block device relative to its wider-stripe-widthcounterpart. Accordingly, the ASL of a given block device within thepseudo-expositive flash device may be tuned to meet endurance andbandwidth targets of a given application.

FIG. 7 illustrates an exemplary generation of a physical block addressin response to an incoming LBA (i.e., logical block address provided bya host in connection/association with a memory access or maintenanceoperation) and block device ID (BDid). As shown, the block device ID isapplied to a block device lookup table (e.g., as described in referenceto FIG. 4) to retrieve information identifying the channels and diesthat constitute the block device (i.e., allocated to the block device)and thus, base die and base channel values that specify startingboundaries within a physical or pseudo-physical die matrix, togetherwith parameters that indicate a number of channels and dies per channelspanned by the block device (Chans, Dies/Chan). In other embodiments,the constituent dies and channels of the block device may be specifiedin other ways (e.g., list of specifically identifiable dies, startingchannel/ending channel together with starting die/ending die, etc.).

In the implementation shown, the block device lookup table also outputsan ASL lookup address to the ASL lookup table. The ASL lookup table, inturn, outputs the recorded ASL parameters (including logical block sizeif implemented as a variable parameter) to an ASL generation logic blockwhich in turn outputs an address-space-layout value that enables theincoming LBA to be decomposed into a set of hierarchical “sub-address”values, one sub-address for the group of elements at each respectivelevel of the structural hierarchy within the flash device (e.g., channelsub-address, die sub-address, erase-unit sub-address, page sub-address).

This LBA decomposition into sub-addresses is easiest understood byconsidering the special case in which each sub-address maps to apower-of-two number of corresponding hierarchical elements (e.g., 2⁴channels, 2³ dies per channel, 2¹¹ erase units per die, 2⁸ pages pererase unit) as, in that case, the ASL indicates, in effect, the locationof discrete bit fields within the incoming LBA that correspond torespective levels of the structural hierarchy. Moreover, as shown in theshaded examples of ASLs for block devices 0-4 (BD0-BD4) at, the ASL alsoidentifies the position of each discrete bit field within the ASL—aconsequence of the sequential LBA assignment parameters described abovein reference to FIGS. 4 and 5. Considering the ASL for block device 0,for example, it can be seen that, except for the page offset fixed atthe lowest bits of the incoming LBA, the channel sub-address fieldconstitutes the least significant field of the BD0 ASL, followed by thedie sub-address field, the erase-unit sub-address field and then thepage-address sub-address field. Accordingly, assuming for example thatthe page offset is a two-bit field such that four logical blocks arestored within each flash page, then every fourth LBA in a linearsequence of LBAs will be written within a die on a different channelwith modulo wrap around to an initial channel after a die on the finalchannel has been written. By contrast, the sub-address (or field) orderin the block 1 ASL is exactly the opposite that of block device 0(excepting the least-significant page offset field), such that pages aresequenced first, then erase-units, then dies, and then channels at theend.

Continuing with FIG. 7, as the ASL indicates how the individualhierarchical sub-address fields have been packed or encoded within theincoming logical block address, that value is supplied topseudo-physical address generator and used therein to extract (or unpackor decode) or otherwise recover the individual hierarchicalsub-addresses. In the particular example shown, the erase-unitsub-address (EUvirt) is assumed to be a virtual address (e.g., resolvingbetween 2048 possible erase units within a die having, in actuality,some larger number of erase units) which is therefore supplied to asub-address translation table (or lookup table) for un-mapping to thephysical address of the erase unit to be accessed. In the particularexample shown, the remaining sub-address fields are assumed to bephysical addresses corresponding to respective levels of the structuralhierarchy so that those sub-address fields, together with the output ofthe translated erase-unit address (EUphys) constitute a physical blockaddress that may be supplied to address decoding logic to issue thenecessary control and enable signals to effect access to the specifiedpage of flash storage (at the offset specified by the page offset).

In an alternative embodiment, the channel, die, page and page-offsetsub-addresses (or any subset thereof) recovered from the LBA are alsovirtual addresses and thus supplied to respective sub-addresstranslation or lookup tables (Ch LUT, Die LUT, Pg LUT, PO LUT) to obtainor generate the corresponding physical sub-addresses. More generally,any or all of the sub-address fields (including the page offset)recovered from the inbound LBA may be virtual addresses that areconverted to physical addresses through translation/lookup operations.Also, two or more of the lookup tables for different hierarchical levelsmay be merged or combined. For example, the channel and die lookuptables may be merged to enable a two-dimensional lookup, thusvirtualizing the die array as a whole and enabling any die on anychannel to be interchanged (i.e., through virtual-to-physical addresstranslation) with any other die on the same channel or any otherchannel.

A conceptual view of an erase-unit virtual-to-physical (V2P) translationtable implementation is shown conceptually in the bottom portion of FIG.7 to demonstrate that a channel and die, selected by one of numerousconstituent erase-unit lookup tables (or translation tables) is selectedto yield the physical erase-unit address (i.e., to provide forindependent erase-unit mapping from die to die). Various otherimplementations may be deployed in alternative embodiments.

A number of points bear emphasis in view of FIG. 7. First, as theindividual sub-address fields are, by virtue of block device and ASLprogramming, modulo-encoded into the incoming logical block address(e.g., as respective bit-fields in the power-of-two example shown), thesub-address values may be deterministically (e.g., algorithmically)recovered from the logical block address extremely quickly andefficiently, avoiding altogether the generally enormous translation datastorage (e.g., tree structures and the like) and extensive translationtimes required for FTL operating modes or implementations. Moreover, thevery presence of the sub-address values within the logical block address(i.e., the logical block address may be viewed as a tuple of thehierarchical sub-address values made possible by the pseudo-physicalspecification of the block device and in which the sub-address valuesthat form the tuple may be re-ordered in a variety of ways and evenfragmented in accordance with the user-specified address space layoutfor a block device or subspace therein) effects an assurance to the hostthat LBAs containing the same sub-addresses down to a given hierarchicallevel will resolve to the same physical resource. For example, LBAscontaining the same channel sub-address are ensured to access a die onthe sub-address-identified channel. Similarly, LBAs containing the samechannel and die sub-addresses are ensured to access an erase unit withinthe same die, and so forth. Moreover, any of the sub-addresses may beencoded into the LBA as a virtual address—that is a sub-address valuethat virtualizes the corresponding (and compartmentalized) physicalresource. Thus, even though a given LBA may be assured to resolve to adesired die (virtual or physical), a virtualized erase-unit sub-addressenables the underlying physical resources within that die to be swappedin and out of service as necessary, all without impacting the linearaddress space and performance perceived by the host.

Also note that the depicted architecture permits address translation tobe reduced to relatively simple operations that can be implemented usinglogic gates and lookup tables, that is, in a manner that can beperformed extremely fast, e.g., on an intra-cycle basis (i.e., in lessthan a clock cycle) or at most using only a handful of clock cycles. Forexample, as will be discussed below, a memory controller can offloadaddress translation to a set of logic gates and prepopulated lookuptables, which can perform address translation without requiringprocessor clock cycles. On-board logic can then update the lookup tablesand/or metadata as appropriate, in a manner that does not encumber I/Olatency. By contradistinction, conventional flash memory tables whichmap a logical page address to potentially any wordline in a flash memorydevice typically require gigabit-size translation tables, withtranslation only being performed using a substantial number of usingprocessor cycles. This architecture, once again, helps streamline I/Olatency, rendering it far more predictable, and reduces addresstranslation time to a negligible quantity.

As discussed above, the exemplary ASL bit-field maps shown in FIG. 7 forthe different block devices assume a power-of-two quantity of physicalresources at each hierarchical level in the flash. In the more generalcase, the resources at any or all physical levels may not bepower-of-two quantities and thus complicate the picture of sub-addressencoding. For example, assuming a 3-channel, 5-die block deviceallocation in which each die contains 2000 pseudo-physical erase unitsand 400 pages per erase unit, the total LBA range (assuming four LBAsper page) will span from 0 to 3*5*2000*400*4 (where ‘*’ denotesmultiplication) and thus from 0 to 48,000,000-1 and thus only part ofthe range represented by the smallest sufficient binary address size2^26. Moreover, because each element of the tuple that constitutes agiven logical block address ranges between 0 and a non-power-of-2maximum (i.e., magnitude of individual tuple elements are not, in allcases, power of 2 numbers), the sub-addresses of different hierarchicalgroups of structures (i.e., channels, dies, erase units, pages, etc.) donot fall within discrete bit fields. Said another way, the power-of-twoASL examples shown in FIG. 7 are a special case of a more generalsolution in which sub-address fields are recovered using moduloarithmetic that reflects the tuple composition of the logical blockaddress. Once again, as depicted, such an architecture can be easilyimplemented entirely or primarily in hardware, thus reducing translationtimes to negligible levels.

FIG. 8 illustrates a conceptual implementation of an address generationmodule that may be used to extract (or recover) sub-address values froman inbound LBA regardless of whether the magnitude of individual tupleelements (i.e., number of channels, number of dies/channel, number oferase units/die, number of pages/erase unit) are power-of-two numbers ornot. As in FIG. 7, any or all of the hierarchical sub-address values,once recovered, may be converted from virtual to physical sub-addressesto collectively form an inbound physical block address (PBA), addingoffsets to reflect the base channel and base die of the block device.For reasons discussed in further detail below, the address generator isimplemented bi-directionally. That is, essentially the same operationsexecuted to yield a PBA from an inbound LBA may be executed in reverseto produce an outbound LBA from a physical block address, thus enablingstatus regarding a given physical resource within the flash device to bereported to the host in terms of the LBA mapping that the hostunderstands. Note that this information can be used to provide contextto a host in order for a host to schedule maintenance or data movement(e.g., data migration or garbage collection) to preselected locations(e.g., where the logical address locations corresponding to the data arematched to the wear of a destination physical location, or where logicaladdress locations having similar age, write frequency and/or readfrequency characteristics are grouped together), as discussed in U.S.Provisional Patent Application No. 62/048,162, entitled “Techniques forData Migration Based On Per-Data Metrics and Memory Degradation,” whichhas been incorporated by reference, as noted earlier.

Referring first to the inbound LBA path, an ASL value (e.g., generatedper the ASL lookup table entry as discussed in reference to FIG. 7) thatdescribes the order of tuple elements within the inbound LBA is suppliedto a pair of steering circuits that yield a set of divisors—products ofindividual tuple elements. More specifically, a magnitude steeringcircuit selects, as magnitudes of the three least significant tupleelements within the inbound LBA (a four-tuple LBA is assumed in thisexample, having as tuple elements, a channel sub-address, diesub-address erase-unit sub-address, and page sub-address in any order),from among the block device configuration settings for structuralelement quantities (i.e, number of allocated channels, number ofallocated dies, number of allocated erase units, number of allocatedpages) to produce divisor values that reflect strides within the LBA(after scaling to account for the page offset—the “scaled LBA” or“offset-scaled LBA”) at which boundaries between different members ofthe same structural group occur. Thus, in the 3-channel, 5-die, 2000erase-unit, 400 page example above, if the tuple elements are orderedfrom most to least significant as channel sub-address, die sub-address,erase-unit sub-address, page sub-address, then the tuple elementmagnitudes will be 400, 2000, and 5, for elements 0, 1 and 2,respectively. Accordingly, the divisors supplied to the divisor steeringlogic will be DM1=200, DM2=400*2000=800,000 andDM3=400*2000*5=4,000,000. The divisor steering logic serves to route thedivisors to the appropriate sub-address generation paths according tothe tuple order specified in the ASL value and thus, continuing with the3 channel, 5 die, 2000 erase unit, 400 page example (i.e., in tuplenotation: “(3, 5, 2000, 400)”), the channel sub-address path willreceive the 400*2000*5 divisor (reflecting that a different channel isspecified at each 4,000,000 increment in the scaled LBA), the diesub-address path will receive the 400*2000 divisor (reflecting that adifferent die is specified at each 800,000 increment in the scaled LBA)and the erase-unit sub-address path will receive the 400 divisor(reflecting that a different erase unit is specified at each 400increment in the scaled LBA). The sub-address path for the leastsignificant tuple element will receive a unity divisor (i.e., ‘1’),reflecting that a different page is specified at each increment of thescaled LBA.

Still referring to the inbound LBA path of FIG. 8, after divisors havebeen applied to the scaled LBA (i.e., integer division as reflected bythe ‘/’ operator in divider logic elements), the results (integer-valuedquotients) are subject to modulo operations (i.e., modulo 400, 2000, 5and 3 for the respective sub-addresses) within the “mod” logic elementsto yield sub-address values which, in the example shown, are assumed tobe virtual sub-address values. As shown, the virtual sub-addresses aresupplied to respective (or joint) lookup/translation modules to generatephysical sub-address counterparts generally as described above inreference to FIG. 7. Any or all of the modulo logic outputs may bephysical sub-address values in alternative embodiments.

Still referring to FIG. 8, the outbound PBA/LBA path is essentially thereverse of the inbound path, converting physical sub-address intovirtual sub-addresses as necessary, multiplying by the steered divisorvalue (now a multiplier and hence a “divisor-multiplier”), and thensumming the multiplied sub-address values to form the four-tuple LBAoutput (merging in the page offset to yield the full-range rather thanoffset-scaled LBA).

FIG. 9 illustrates an exemplary virtualization of erase units within afour-die (2 dies by 2 channels) block device that demonstrates thediscrete defect-free block device exposition enabled by the variouspseudo-expositive flash device embodiments described above. In theexample shown, the ASL is configured similarly to that of the blockdevice in the left-most portion of FIG. 6, but with only single-planeoperation, thus establishing a four-page stripe width. For purposes ofexample, the erase units into which the first sixteen pages of data areenumerated within the depicted erase-unit elements. Further, it isassumed that the erase unit population is larger than the virtual eraseunit address field within each incoming LBA such that some number ofspare erase units is available within each die. Accordingly, upondetermining that the erase unit shown at the bottom right corner of thefourth die is defective, the pseudo-expositive memory controller remapsthe LBA that would otherwise resolve to the defective erase unit to aspare erase unit within the same die. That is, the fifth page of datathat might otherwise be written to the defective erase unit is, throughtranslation of the virtual address field within the incoming LBA to aphysical address field as discussed above, re-directed to the spare pageentirely transparently to the host write requestor. Impending run-timedetected erase-unit failures in dies marked ‘X’ result in similarre-mapping and data move operations (i.e., prior to complete failure ofthe suspect erase units) again permitting the host to perceive the blockdevice as a discrete defect-free storage entity. As discussed above, anyother resources within the structural hierarchy of the flash device(including particularly those most likely to fail), such as dies,planes, pages or even channels may be virtualized and pseudo-exposed tothe host in a manner that preserves one or more spares. In all suchcases, the host may configure the number of structural elements reservedas spares on a flash-device-wide or block-device basis, for example,after requesting and receiving a physical geometry description from theflash device, including a reporting of the number of failed or suspectelements at each level of the structural hierarchy. Such spare-setting(or reserve pool specification) may be carried out, for example, inconnection with block device programming within the block deviceallocator described above in reference to FIG. 2 (e.g., programming thenumber of spares to be reserved, percentage of spares to be reserved,size of virtual address field and thus the allocable number ofstructural elements, etc.) and may be assisted by receiving a defaultreserve pool recommendation from the memory controller (which defaultmay be applied to allocated block devices absent host programming of adifferent setting).

Still referring to FIG. 9, the pseudo-physical host perspective of theblock device remains static and defect free, despite ongoing failureevents (and/or removal of erase units or other virtualized structuralelements from service for wear leveling, garbage collection, scrubbingor other maintenance or operational reasons). The remapping of addressinformation by the memory controller using the identified translationscheme once again helps streamline I/O latency, as discussed earlier.

FIG. 10 demonstrates an exemplary sequence of operations coordinatedbetween the pseudo-expositive flash memory controller and host fileserver to enable host-scheduled maintenance of flash-device-identifiedunits of data, and more specifically segments in this example. Thedevice tracks statistics and environmental status of the memory lookingfor indications that a portion of the data needs maintenance driven by aphysical resource. An example of a maintenance operation might be thatdata has resided on a given EU for a time long enough to approach amaximum estimated safe retention time. An event could be set to triggerat a threshold of safety previously selected by the host andcommunicated to the device. Once that event has been triggered, thedevice translates the physical location though the previous block deviceand ASL map structures in order to identify the segment as a host LBA inwhich the physical resource resides. When the host has received thenotification of the event, along with the associated segment LBAaddress, length, and event type, the host may elect to handle that eventin a number of ways. Host issued instructions may garbage collect, copy,move, or discard data in manner that best benefits the overallefficiency of the system. Those instructions may be mixed into theoverall system scheduling. Note that in contradistinction toconventional approaches (e.g., where an entire address space is garbagecollected, in a device-wide operation delegated to a memory controller)that this approach permits the host to command specific maintenanceoperations to specific hierarchies, for example, by command to performmaintenance on a specific EU; to this effect, the host transmits amaintenance command with a specific block address, which the memorycontroller uses to perform maintenance on a host-selected orhost-specified address range, conveyed by or in connection with themaintenance command. Once data has been relocated or discarded, asegment of interest may be erased.

FIG. 11 illustrates a forward (and reverse) compatibility betweensuccessive flash generations that is enabled by the pseudo-expositiveflash device in combination with the block device allocator discussedabove in reference to FIG. 2. Assume, for example, that the block deviceallocator was initially executed in connection with a generation ‘A’flash device having a page size ‘K’, to configure a block device havinga four-page stripe size. For ease of illustration, the K-sized pages areassumed sufficiently large that each erase unit contains only two pages,and that each host segment spans four erase units. Thus, a first hostsegment (“Seg0”) is striped across the four dies of the block device andfills erase unit 0 within each die, being stored in eight pages total(pages 0-7), while a second host segment (“Seg1”) is stored within eraseunit 1 of the four dies and thus in pages 8-15. Assume as aforward-compatibility example, that a next generation “GenB” flashdevice is to be deployed in combination with or in place of a “GenA”flash device, and that page size has doubled to 2K so that the overallflash device capacity has doubled in GenB (i.e., there are the samenumber of pages per erase unit, erase units per die, dies per channeland channels as in the GenA device so that the doubled page size doublesthe capacity of every element in the structural hierarchy). In thiscase, assuming further that the page read and write times (t_(READ) andt_(PROG)) have also doubled between the two flash device generations,then the user/designer may re-run the block device allocator with anunchanged set of block device specifications (i.e., in terms of read andwrite bandwidth, at least) to yield a block device allocation and ASLwithin the GenB device that matches the performance characteristics ofthe counterpart block device within the GenA flash device, albeit atdouble capacity. For example, as shown in FIG. 11, the block deviceallocator may define the ASL such that the stripe width still spans fourpages (i.e., a page within each of the four allocated dies) so that thereduced write bandwidth (due to the 2× higher GenB tProg) is offset bythe doubled page size. Thus, the two segments previously stored withinthe GenA drive may be written to the GenB drive with the same bandwidth,but now occupy only half the number of erase units (as their capacityhas doubled) as in the GenA storage. Note that Seg1 may, throughdifferent ASL specification, be stored within dies 0 and 1 of the GenBflash device instead of dies 2 and 3, thus raising endurance of theflash device at the cost of reduced write bandwidth. Changing theexample so that the tPROG and tREAD times remain unchanged in the GenBdevice, then the alternative segment 1 storage destination (i.e.,alternative ASL specification) within dies 0 and 1 may be preferable, asthe two-die stripe size in the GenB device would yield the same writebandwidth as the four-die stripe size in the GenA device. That is,sequencing more narrowly through dies 0 and 1 (instead of more broadlyacross all four dies) will yield a higher endurance result withapproximately the same read/write bandwidth. As discussed above, the ASLmay be further tuned to yield a desired balance between endurance andI/O bandwidth.

FIG. 12 illustrates more detail regarding pseudo-physical geometryexport options within a flash device having multi-plane flash dies. Inthe particular example shown, it is assumed that each die within theflash device, exemplified by “Die 0” includes N erase units distributedevenly between even and odd erase-unit planes (i.e., N/2 erase units perplane). In general, the die may be switched on the fly, according toincoming command sequences, between single-plane and multi-planeoperation. Once again, therefore, the same die (“Die 0”) is illustratedin duplicate to discuss multiple alternate configurations; thepseudo-expositive memory controller may operate the die in one ofseveral different pseudo-physical modes including, an opportunisticsingle-plane mode, an exclusively multi-plane mode, and a range-splithybrid mode in which a portion of the pseudo-physical or physical eraseunits within the die are operated in opportunistic single-plane mode andanother portion of the erase units are operated in exclusivelymulti-plane mode. In each of the pseudo-physical operating modes, theerase unit field of an incoming LBA resolves to fewer than N eraseunits. For example, the erase unit field resolves to any one of M eraseunits in single-plane mode, to any one of M/2 double-width erase units(i.e., logical combination of paired erase units drawn respectively fromthe odd and even planes) in dual plane mode and any one of Ksingle-plane erase units or J double-width erase units in the hybridsplit-range mode, where M<N and K+2J<N.

In opportunistic single-plane mode, the flash die is generally accessedone erase-unit at a time (or more accurately one page within one eraseunit at a time, for reads and writes), occasionally executing adual-plane command where a pair of successive accesses are directedrespectively to erase units within the odd and even planes. By contrast,when configured for dual plane mode, each host requested memory accessis translated by the memory controller into a dual plane commandsequence that accesses a matched pair of erase units within each of theodd and even planes (i.e., one odd-plane erase unit and a counterparteven-plane erase unit). Accordingly, the page registers and erase unitsare viewed, from a pseudo-physical perspective, as double-width elementsas compared to individual page registers and erase units in single-planemode. Moreover, the total number of erase units is halved (by theeffective merging of the two planes) so that the pseudo physical addressrange is reconfigured to account for half the number of virtuallyaddressable erase units within the die, and double the number of logicalblocks per page.

Still referring to FIG. 12, the split-range hybrid mode, due to its dualrange-divided configuration, permits single-plane commands andopportunistic multi-plane command sequences within the single-planeaddress range (i.e., logical addresses that resolve to erase units belowpseudo-physical erase unit K), and requires exclusively multi-planecommand sequences within the dual-plane address range. In alternativeembodiments, the ordering of the dual-plane and single-plane ranges maybe reversed (i.e., dual-plane at lower rather than higher-orderedaddresses) and the total number of different ranges may be greater thanthe two shown.

FIG. 13 illustrates exemplary configurations of a memory die (“Die 0”)in various embodiments. Once again, “Die 0” is shown in multiple,alternate configurations. A defective erase unit detected within a givenplane of a multi-plane capable die may be mapped (logically replaced) byan otherwise reserved erase unit within the same plane as shown in thetwo pseudo-physical views of die zero. This enables opportunisticmulti-plane operations to continue when the die is configured forsingle-plane operation, and permits multi-plane operations to proceedentirely transparently to the host.

FIG. 14 illustrates one embodiment of various techniques illustratedabove, implemented as a memory controller integrated circuit (IC). Asnoted by the figure, in one version, such an IC can be implemented as afield-programmable gate array (FPGA) having logic gates and lookuptables (e.g., circuitry) configured to provide fast address translationas discussed above; this is not required for all embodiments, and inother contemplated embodiments, a memory controller can be implementedas a special purpose processor or other preconfigured integratedcircuit, with or without the use of support software (e.g., firmware).As depicted by the figure, the IC includes a host interface, which forexample, uses a standard communication interface framework (e.g.,non-volatile memory express, or NVME or Serial ATA) for packetizedcommunications, where packets comprise commands, address and data, asappropriate; note that this interface can be shared for commandsdirected to different channels or devices, i.e., the commands and datato respective channels and/or dies are typically multiplexed or “MUX'ed”over this channel for exchange between the memory controller IC and thehost. Similarly, as discussed earlier, the memory controller features aflash interface, whereby one or more channels are used to communicatewith one or more memory devices. Each channel can consist of a set oflinks for address and/or data (e.g., for packetized communication ofaddress and data over a set of eight links), or can feature separatedaddress and data busses (as depicted by the optional, dashed-line linksdesignated for “data”). The memory controller IC also features hardwaretranslation circuitry as mentioned (e.g., with ASL parameters being usedto configure address wrapping and distinction between hierarchicallevels and with LUTs used for address mapping at selected orpredetermined levels of the hierarchy) and buffers for bufferingcommands and/or data transfer as appropriate. In addition, the memorycontroller IC in this embodiment can include supervisory logic (e.g.,“circuitry”) for responding to host queries, performing LUT updates, andconfiguring parameters as appropriate, as well as for controllingsequencing and maintenance operations in memory, and/or for implementinga block-device designer and allocator section as discussed earlier inconnection with FIG. 2, and block device configuration elements, asappropriate. In one embodiment, some or all of these functions can beimplemented in the form of instructions stored on non-transitorymachine-readable media (e.g., firmware or software, for controlling oneor more cores or processors, as part of such circuitry).

It should be noted that, as a step in their fabrication or otherreification, the various circuits disclosed herein may be describedusing computer aided design tools and expressed (or represented) as dataand/or instructions embodied in various computer-readable media, interms of their behavioral, register transfer, logic component,transistor, layout geometries, and/or other characteristics. Formats offiles and other objects in which such circuit expressions may beimplemented include, but are not limited to, formats supportingbehavioral languages such as C, Verilog, and VHDL, formats supportingregister level description languages like RTL, and formats supportinggeometry description languages such as GDSII, GDSIII, GDSIV, CIF, MEBESand any other suitable formats and languages. Computer-readable media inwhich such formatted data and/or instructions may be embodied include,but are not limited to, computer storage media in various forms (e.g.,optical, magnetic or semiconductor storage media, whether independentlydistributed in that manner, or stored “in situ” in an operating system).

When received within a computer system via one or more computer-readablemedia, such data and/or instruction-based expressions of the abovedescribed circuits may be processed by a processing entity (e.g., one ormore processors) within the computer system in conjunction withexecution of one or more other computer programs including, withoutlimitation, net-list generation programs, place and route programs andthe like, to generate a representation or image of a physicalmanifestation of such circuits. Such representation or image maythereafter be used in device fabrication, for example, by enablinggeneration of one or more masks that are used to form various componentsof the circuits in a device fabrication process. Any of the variousmethods and operational sequences herein may likewise be recorded as oneor more sequences of instructions on a computer-readable medium and maybe executed on a computing device to effectuate the disclosed methodand/or operational sequence. Also, as noted, many of the techniquesdescribed herein can be employed in an apparatus, a method, anintegrated circuit, a system on-chip, a memory device, a memorycontroller, a host processor, as a circuit description (i.e., thatcontains data structures defining fabrication parameters for aprocessor, integrated circuit, device, or components of any of thesethings), as instructions stored on machine-readable media (e.g.,firmware or software intended for execution on one or more generalpurpose machines), or as combinations of these things. In the case ofsoftware or other instructional logic, the instructions are typicallywritten or designed in a manner that has certain structure(architectural features) such that, when they are ultimately executed,they cause the one or more general purpose machines or hardware tobehave as special purpose machines, having structure configured by theinstructions to necessarily perform certain described tasks.“Non-transitory machine-readable media” as used herein means anytangible (i.e., physical) storage medium, irrespective of how data onthat medium is stored, including without limitation, random accessmemory, hard disk memory, optical memory, a floppy disk or CD, serverstorage, volatile memory and other forms of tangible storage whereinstructions may subsequently be retrieved by a machine. Themachine-readable media can be in standalone form (e.g., a program disk)or embodied as part of a larger mechanism, for example, a storage drive,CPU, laptop computer, portable or mobile device, server, data center,“blade” device, subsystem, electronics “card,” storage device, network,or other set of one or more other forms of devices. The instructions canbe implemented in different formats, for example, as metadata that whencalled is effective to invoke a certain action, as “JAVA” code orscripting, as code written in a specific programming language (e.g., asC++ code), as a processor-specific instruction set, or in some otherform; the instructions can also be executed by the same processor ordifferent processors, depending on embodiment. For example, in oneimplementation, instructions on non-transitory machine-readable mediacan be executed by a single computer and, in other cases as noted, canbe stored and/or executed on a distributed basis, e.g., using one ormore servers, web clients, or application-specific devices. Eachfunction mentioned in the disclosure or FIGS. can be implemented as partof a combined program or as a standalone module, either stored togetheron a single media expression (e.g., single floppy disk) or on multiple,separate storage devices. The same is also true for a circuitdescription for fabricating cores, processors, devices or circuitsdescribed herein, i.e., the result of creating a design can be stored innon-transitory machine-readable media for temporary or permanent use,either on the same machine or for use on one or more other machines; forexample, a circuit description or software can be generated using afirst machine, and then stored for transfer to a printer ormanufacturing device, e.g., for download via the internet (or anothernetwork) or for manual transport (e.g., via a transport media such as adigital versatile disk, or “DVD”) for use on another machine. Throughoutthis disclosure, various processes will be described, any of which cangenerally be implemented as instructional logic (instructions stored onnon-transitory machine-readable media), or as “circuitry,” i.e.,hardware logic or a combination of hardware logic and instructionallogic. Depending on product design, such products can be fabricated tobe in saleable form, or as a preparatory step for other processing thatwill ultimately create finished products for sale, distribution,exportation or importation.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, any of the specific numbers ofbits, signal path widths, signaling or operating frequencies, devicegeometries and numbers of hierarchical structural elements (e.g.,channels, dies, planes, erase units, pages, etc.), component circuits ordevices and the like may be different from those described above inalternative embodiments. Additionally, links or other interconnectionbetween integrated circuit devices or internal circuit elements orblocks may be shown as buses or as single signal lines. Each of thebuses may alternatively be a single signal line, and each of the singlesignal lines may alternatively be buses. Signals and signaling links,however shown or described, may be single-ended or differential. Asignal driving circuit is said to “output” a signal to a signalreceiving circuit when the signal driving circuit asserts (or deasserts,if explicitly stated or indicated by context) the signal on a signalline coupled between the signal driving and signal receiving circuits.The term “coupled” is used herein to express a direct connection as wellas a connection through one or more intervening circuits or structures.Device “programming” may include, for example and without limitation,loading a control value into a register or other storage circuit withinan integrated circuit device in response to a host instruction (and thuscontrolling an operational aspect of the device and/or establishing adevice configuration) or through a one-time programming operation (e.g.,blowing fuses within a configuration circuit during device production),and/or connecting one or more selected pins or other contact structuresof the device to reference voltage lines (also referred to as strapping)to establish a particular device configuration or operation aspect ofthe device. The terms “exemplary” and “embodiment” are used to expressan example, not a preference or requirement.

While the invention has been described with reference to specificembodiments thereof, it will be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope. For example, features or aspects of any of the embodimentsmay be applied in combination with any other of the embodimentsdisclosed herein and/or in materials incorporated by reference or inplace of counterpart features or aspects thereof. Accordingly, thespecification and drawings are to be regarded in an illustrative ratherthan a restrictive sense.

What is claimed is:
 1. An apparatus to transmit data access requests toa memory controller for flash memory, the apparatus comprising:circuitry to assign non-overlapping logical address ranges to respectivevirtual block devices defined with respect to the flash memory, whereineach one of the respective virtual block devices is characterized byassociated performance characteristics comprising a minimum timerequired to program a page of memory cells in the flash memory, aminimum time required to erase a block of memory cells in the flashmemory, and a minimum time required to read a page of memory cells inthe flash memory; circuitry to receive from the memory controller, on anindependent basis for each one of the respective virtual block devices,information identifying the need for a maintenance operation, themaintenance operation for each one of the respective virtual blockdevices comprising at least one of erase of an erase unit in the flashmemory corresponding to the one of the respective virtual block devicesor a data relocation operation for data stored in the flash memorycorresponding to the one of the respective virtual block devices; andcircuitry to schedule issuance of commands to the memory controller, thecommands to control the performance of maintenance operations inrespective virtual block devices, in a manner such that the schedulingof commands to perform a maintenance operation in a first one of therespective virtual block devices is unconstrained by the performancecharacteristics associated with a second one of the respective virtualblock devices.
 2. The apparatus of claim 1, wherein the flash memoryincludes, for each of the virtual block devices, a set of access-enabledstructural elements and a set of access-reserved structural elements,each access-enabled structural element and each access-reservedstructural element having an associated physical address, and whereinthe apparatus is to, independently, for each of the respective virtualblock devices: identify a size associated with a corresponding set ofaccess-enabled structural elements; assign the non-overlapping logicaladdress range for the respective virtual block device in dependence onthe size, such that the non-overlapping address range for the respectivevirtual block device represents a subset of physical flash memory spaceassociated with the respective virtual block device; andprogrammably-configure the memory controller to map the non-overlappingaddress range for the respective virtual block device to thecorresponding set of access-enabled structural elements.
 3. Theapparatus of claim 2, wherein the flash memory includes flash memorydies, and wherein: the apparatus is to, for each of the respectivevirtual block devices, programmably-configure the memory controller tomap the non-overlapping address ranges to a configurable number of theflash memory dies, the configurable number being one or more; and theconfigurable number is free to be different for a first one of therespective virtual block devices than for a second one of the respectivevirtual block devices.
 4. The apparatus of claim 3, wherein theapparatus is further operable to, for at least one of the respectivevirtual block devices, programmably-configure the memory controller suchthat the access-enabled set of structural elements for the at least oneof the virtual block devices comprises one or more discrete flash memorydies, and such that the reserved set of structural elements for the atleast one comprises one or more discrete flash memory dies.
 5. Theapparatus of claim 1, wherein the flash memory is coupled to the memorycontroller by multiple independent communication channels, and wherein:the apparatus is to identify a number of channels to be associated witheach respective virtual block device, the number being one or more, andprogrammably-configure the memory controller to map the logical addressranges for each respective virtual block device to the number ofchannels identified for the respective virtual block device; and thenumbers for the respective virtual block devices can be different fromone another.
 6. The apparatus of claim 1, wherein the flash memorycomprises first structural elements and second structural elements, andwherein the apparatus further comprises circuitry to program the memorycontroller to operate at least one of the respective virtual blockdevices in a selective one of a first mode and a second mode, wherein inthe first mode, successive writes are to be directed by the memorycontroller to successive ones of the first structural elements, andwherein in the second mode, successive writes are to be directed by thememory controller to successive ones of the second structural elementswithin a single one of the second structural elements.
 7. The apparatusof claim 6, wherein the first structural elements comprise erase unitsof flash memory and the second structural elements comprise pages offlash memory.
 8. The apparatus of claim 6, wherein the informationidentifying the need for a maintenance operation comprises at least onelogical address associated with one of the respective virtual blockdevices, and wherein: the circuitry to schedule issuance of commands,responsive to receipt of said information, is to schedule (a) movementof data corresponding to the at least one logical address to a differentphysical memory location associated with the one of the respectivevirtual block devices, and (b) erase of a physical memory location whichpreviously stored the data; and the circuitry to schedule issuance ofcommands is further to sequentially issue commands to perform thescheduled movement of the data and the erase of the physical memorylocation in dependence on the performance characteristics associatedwith the one of the respective virtual block devices.
 9. The apparatusof claim 8, wherein the one of the respective virtual block devices is afirst virtual block device, and wherein the circuitry to scheduleissuance of commands is operable to issue one of a data read or datawrite command to the memory controller for performance in a secondvirtual block device different than the first virtual block device, thedata read or write command to be performed concurrently with performanceof at least one of the sequentially-issued commands and irrespective ofthe performance characteristics of the first virtual block device. 10.An apparatus comprising instructions stored on non-transitorymachine-readable media, said apparatus to cause an electronic system totransmit data access requests to a memory controller for flash memory,said instructions when executed to cause at least one integrated circuitprocessor of the electronic system: assign non-overlapping logicaladdress ranges to respective virtual block devices defined with respectto the flash memory, wherein each one of the respective virtual blockdevices is characterized by associated performance characteristicscomprising a minimum time required to program a page of memory cells inthe flash memory, a minimum time required to erase a block of memorycells in the flash memory, and a minimum time required to read a page ofmemory cells in the flash memory; receive from the memory controller, onan independent basis for each one of the respective virtual blockdevices, information identifying the need for a maintenance operation,the maintenance operation for each one of the respective virtual blockdevices comprising at least one of erase of an erase unit in the flashmemory corresponding to the one of the respective virtual block devicesor a data relocation operation for data stored in the flash memorycorresponding to the one of the respective virtual block devices; andschedule issuance of commands to the memory controller to performmaintenance operations in respective virtual block devices, in a mannersuch that the scheduling of commands to perform a maintenance operationin a first one of the respective virtual block devices is unconstrainedby the performance characteristics associated with a second one of therespective virtual block devices.
 11. The apparatus of claim 10, whereinthe flash memory includes, for each of the virtual block devices, a setof access-enabled structural elements and a set of access-reservedstructural elements, each access-enabled structural element and eachaccess-reserved structural element having an associated physicaladdress, and wherein the apparatus further comprises instructions that,when executed, are to cause the at least one integrated circuitprocessor to, independently, for each of the respective virtual blockdevices: identify a size associated with a corresponding set ofaccess-enabled structural elements; assign the non-overlapping logicaladdress range for the respective virtual block device in dependence onthe size, such that the non-overlapping address range for the respectivevirtual block device represents a subset of physical flash memory spaceassociated with the respective virtual block device; andprogrammably-configure the memory controller to map the non-overlappingaddress range for the respective virtual block device to thecorresponding set of access-enabled structural elements.
 12. Theapparatus of claim 11, wherein the flash memory includes flash memorydies, and wherein: the apparatus further comprises instructions that,when executed, are to cause the at least one integrated circuitprocessor to, independently, for each of the respective virtual blockdevices, programmably-configure the memory controller to map thenon-overlapping address ranges to a configurable number of the flashmemory dies, the configurable number being one or more; and theconfigurable number is free to be different for a first one of therespective virtual block devices than for a second one of the respectivevirtual block devices.
 13. The apparatus of claim 12, wherein theapparatus further comprises instructions that, when executed, are tocause the at least one integrated circuit processor to, for at least oneof the respective virtual block devices, programmably-configure thememory controller such that the access-enabled set of structuralelements for the at least one of the virtual block devices comprises oneor more discrete flash memory dies, and such that the reserved set ofstructural elements for the at least one comprises one or more discreteflash memory dies.
 14. The apparatus of claim 10, wherein the flashmemory is coupled to the memory controller by multiple independentcommunication channels, and wherein: the apparatus further comprisesinstructions that, when executed, are to cause the at least oneintegrated circuit processor to identify a number of channels to beassociated with each respective virtual block device, the number beingone or more, and programmably-configure the memory controller to map thelogical address ranges for each respective virtual block device to thenumber of channels identified for the respective virtual block device;and the numbers for the respective virtual block devices can bedifferent from one another.
 15. The apparatus of claim 10, wherein theflash memory comprises first structural elements and second structuralelements, and wherein the apparatus further comprises instructions that,when executed, are to cause the at least one integrated circuitprocessor to program the memory controller to operate at least one ofthe respective virtual block devices in a selective one of a first modeand a second mode, wherein in the first mode, successive writes are tobe directed by the memory controller to successive ones of the firststructural elements, and wherein in the second mode, successive writesare to be directed by the memory controller to successive ones of thesecond structural elements within a single one of the second structuralelements.
 16. The apparatus of claim 15, wherein the first structuralelements comprise erase units of flash memory and the second structuralelements comprise pages of flash memory.
 17. The apparatus of claim 15,wherein the information identifying the need for a maintenance operationcomprises at least one logical address associated with one of therespective virtual block devices, and wherein the apparatus furthercomprises instructions that, when executed, are to cause the at leastone integrated circuit processor to: schedule issuance of commandsresponsive to receipt of said information to perform (a) movement ofdata corresponding to the at least one logical address to a differentphysical memory location associated with the one of the respectivevirtual block devices, and (b) erase of a physical memory location whichpreviously stored the data; and sequentially issue the commands toperform the scheduled movement of the data and the erase of the physicalmemory location in dependence on the performance characteristicsassociated with the one of the respective virtual block devices.
 18. Theapparatus of claim 17, wherein the one of the respective virtual blockdevices is a first virtual block device, and wherein the apparatusfurther comprises instructions that, when executed, are to cause the atleast one integrated circuit processor to issue one of a data read ordata write command to the memory controller for performance in a secondvirtual block device different than the first virtual block device, thedata read or write command to be performed concurrently with performanceof at least one of the sequentially-issued commands and irrespective ofthe performance characteristics of the first virtual block device. 19.An apparatus to transmit data access requests to a memory controller forflash memory, the flash memory comprising flash memory dies controlledin common by the memory controller, the apparatus comprising: circuitryto assign non-overlapping logical address ranges to respective virtualblock devices defined with respect to the flash memory, wherein each oneof the respective virtual block devices is associated with a respectivesubset of one or more of the flash memory dies on a mutually-exclusivebasis relative to each other and wherein each one of the respectivevirtual block devices is characterized by associated performancecharacteristics comprising a minimum time required to program a page ofmemory cells in the flash memory, a minimum time required to erase ablock of memory cells in the flash memory, and a minimum time requiredto read a page of memory cells in the flash memory; circuitry to receivefrom the memory controller, on an independent basis for each one of therespective virtual block devices, information identifying the need for amaintenance operation in a particular die of the respective subset ofthe flash memory dies, the maintenance operation for each one of therespective virtual block devices comprising at least one of erase of anerase unit in the particular die or a data relocation operation for datastored in the particular die; and circuitry to schedule issuance ofcommands to the memory controller, the commands to control performanceof maintenance operations, in a manner such that the scheduling ofcommands to perform a maintenance operation in a first one of therespective virtual block devices is unconstrained by the performancecharacteristics associated with a second one of the respective virtualblock devices.